Modifier 91 — “Repeat Clinical Diagnostic Laboratory Test” — is appended when the exact same CPT laboratory test is performed more than once for the same patient on the same day because serial, medically necessary monitoring is required. The modifier indicates the repeated result is subsequently reportable and not a repeat performed to correct a specimen, equipment error, or merely to confirm a prior result. CMS, AMA guidance, and Medicare contractors (e.g., Noridian) all emphasize that 91 applies only when the repeat is clinically necessary and the procedure code does not already describe a timed series of tests.

Proper use of modifier 91 allows repeat laboratory tests to be considered for payment under the Clinical Laboratory Fee Schedule and can bypass some automated edits that would otherwise deny duplicate testing, provided the claim is supported by clinical documentation showing medical necessity. Inappropriate use—such as reruns due to specimen/equipment errors, confirmation of results, or when another CPT code already describes a series—can lead to denials; payers and Medicare contractors explicitly require documentation of the clinical rationale. Examples include serial potassium measurements during DKA (e.g., 84132, 84132-91, 84132-91) and multiple arterial blood gas analyses the same day (e.g., 82803 with subsequent 82803-91 entries).